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systemd is a system and service manager for Linux operating systems. When run as
first process on boot (as PID 1), it acts as init system that brings up and
maintains userspace services.

For compatibility with SysV, if systemd is called as init
and a PID that is not 1, it will execute telinit and pass all command
line arguments unmodified. That means init and telinit are
mostly equivalent when invoked from normal login sessions. See
telinit(8) for more information.

When run as a system instance, systemd interprets the
configuration file system.conf and the files in system.conf.d directories;
when run as a user instance, systemd interprets the configuration file
user.conf and the files in user.conf.d directories. See
systemd-system.conf(5) for more information.

Dump exposed bus properties. This outputs a terse but
complete list of properties exposed to dbus.

--unit=

Set default unit to activate on startup. If not
specified, defaults to default.target.

--system, --user

For --system, tell systemd to run a system
instance, even if the process ID is not 1, i.e. systemd is not run as init
process. --user does the opposite, running a user instance even if the
process ID is 1. Normally, it should not be necessary to pass these options,
as systemd automatically detects the mode it is started in. These options are
hence of little use except for debugging. Note that it is not supported
booting and maintaining a full system with systemd running in --system
mode, but PID not 1. In practice, passing --system explicitly is only
useful in conjunction with --test.

--dump-core

Enable core dumping on crash. This switch has no effect
when running as user instance. This setting may also be enabled during boot on
the kernel command line via the systemd.dump_core= option, see
below.

--crash-vt=VT

Switch to a specific virtual console (VT) on crash. Takes
a positive integer in the range 1–63, or a boolean argument. If an
integer is passed, selects which VT to switch to. If yes, the VT kernel
messages are written to is selected. If no, no VT switch is attempted.
This switch has no effect when running as user instance. This setting may also
be enabled during boot, on the kernel command line via the
systemd.crash_vt= option, see below.

--crash-shell

Run a shell on crash. This switch has no effect when
running as user instance. This setting may also be enabled during boot, on the
kernel command line via the systemd.crash_shell= option, see
below.

--crash-reboot

Automatically reboot the system on crash. This switch has
no effect when running as user instance. This setting may also be enabled
during boot, on the kernel command line via the systemd.crash_reboot=
option, see below.

--confirm-spawn

Ask for confirmation when spawning processes. This switch
has no effect when run as user instance.

--show-status=

Takes a boolean argument or the special value
auto. If on, terse unit status information is shown on the console
during boot-up and shutdown. If off, no such status information is shown. If
set to auto behavior is similar to off, except that it is automatically
switched to on, as soon as the first unit failure or significant boot delay is
encountered. This switch has no effect when invoked as user instance. If
specified, overrides both the kernel command line setting
systemd.show_status= (see below) and the configuration file option
ShowStatus=, see systemd-system.conf(5).

--log-target=

Set log target. Argument must be one of console,
journal, kmsg, journal-or-kmsg, null.

Highlight important log messages. Argument is a boolean
value. If the argument is omitted, it defaults to true.

--log-location=

Include code location in log messages. This is mostly
relevant for debugging purposes. Argument is a boolean value. If the argument
is omitted it defaults to true.

--default-standard-output=,
--default-standard-error=

Sets the default output or error output for all services
and sockets, respectively. That is, controls the default for
StandardOutput= and StandardError= (see systemd.exec(5)
for details). Takes one of inherit, null, tty,
journal, journal+console, syslog, syslog+console,
kmsg, kmsg+console. If the argument is omitted
--default-standard-output= defaults to journal and
--default-standard-error= to inherit.

--machine-id=

Override the machine-id set on the hard drive, useful for
network booting or for containers. May not be set to all zeros.

--service-watchdogs=

Globally enable/disable all service watchdog timeouts and
emergency actions. This setting may also be specified during boot, on the
kernel command line via the systemd.service_watchdogs= option, see
below. Defaults to enabled.

systemd provides a dependency system between various entities called
"units" of 11 different types. Units encapsulate various objects
that are relevant for system boot-up and maintenance. The majority of units
are configured in unit configuration files, whose syntax and basic set of
options is described in systemd.unit(5), however some are created
automatically from other configuration, dynamically from system state or
programmatically at runtime. Units may be "active" (meaning started,
bound, plugged in, ..., depending on the unit type, see below), or
"inactive" (meaning stopped, unbound, unplugged, ...), as well as in
the process of being activated or deactivated, i.e. between the two states
(these states are called "activating", "deactivating"). A
special "failed" state is available as well, which is very similar
to "inactive" and is entered when the service failed in some way
(process returned error code on exit, or crashed, an operation timed out, or
after too many restarts). If this state is entered, the cause will be logged,
for later reference. Note that the various unit types may have a number of
additional substates, which are mapped to the five generalized unit states
described here.

The following unit types are available:

1.Service units, which start and control daemons and the
processes they consist of. For details, see systemd.service(5).

2.Socket units, which encapsulate local IPC or network
sockets in the system, useful for socket-based activation. For details about
socket units, see systemd.socket(5), for details on socket-based
activation and other forms of activation, see daemon(7).

3.Target units are useful to group units, or provide
well-known synchronization points during boot-up, see
systemd.target(5).

4.Device units expose kernel devices in systemd and may
be used to implement device-based activation. For details, see
systemd.device(5).

5.Mount units control mount points in the file system,
for details see systemd.mount(5).

6.Automount units provide automount capabilities, for
on-demand mounting of file systems as well as parallelized boot-up. See
systemd.automount(5).

7.Timer units are useful for triggering activation of
other units based on timers. You may find details in
systemd.timer(5).

8.Swap units are very similar to mount units and
encapsulate memory swap partitions or files of the operating system. They are
described in systemd.swap(5).

9.Path units may be used to activate other services when
file system objects change or are modified. See systemd.path(5).

10.Slice units may be used to group units which manage
system processes (such as service and scope units) in a hierarchical tree for
resource management purposes. See systemd.slice(5).

11.Scope units are similar to service units, but manage
foreign processes instead of starting them as well. See
systemd.scope(5).

Units are named as their configuration files. Some units have
special semantics. A detailed list is available in
systemd.special(7).

systemd knows various kinds of dependencies, including positive
and negative requirement dependencies (i.e. Requires= and
Conflicts=) as well as ordering dependencies (After= and
Before=). NB: ordering and requirement dependencies are orthogonal.
If only a requirement dependency exists between two units (e.g. foo.service
requires bar.service), but no ordering dependency (e.g. foo.service after
bar.service) and both are requested to start, they will be started in
parallel. It is a common pattern that both requirement and ordering
dependencies are placed between two units. Also note that the majority of
dependencies are implicitly created and maintained by systemd. In most
cases, it should be unnecessary to declare additional dependencies manually,
however it is possible to do this.

Application programs and units (via dependencies) may request
state changes of units. In systemd, these requests are encapsulated as
'jobs' and maintained in a job queue. Jobs may succeed or can fail, their
execution is ordered based on the ordering dependencies of the units they
have been scheduled for.

On boot systemd activates the target unit default.target whose job
is to activate on-boot services and other on-boot units by pulling them in
via dependencies. Usually, the unit name is just an alias (symlink) for
either graphical.target (for fully-featured boots into the UI) or
multi-user.target (for limited console-only boots for use in embedded or
server environments, or similar; a subset of graphical.target). However, it
is at the discretion of the administrator to configure it as an alias to any
other target unit. See systemd.special(7) for details about these
target units.

systemd only keeps a minimal set of units loaded into memory.
Specifically, the only units that are kept loaded into memory are those for
which at least one of the following conditions is true:

1.It is in an active, activating, deactivating or failed
state (i.e. in any unit state except for "inactive")

2.It has a job queued for it

3.It is a dependency of some sort of at least one other
unit that is loaded into memory

4.It has some form of resource still allocated (e.g. a
service unit that is inactive but for which a process is still lingering that
ignored the request to be terminated)

5.It has been pinned into memory programmatically by a
D-Bus call

systemd will automatically and implicitly load units from disk
— if they are not loaded yet — as soon as operations are
requested for them. Thus, in many respects, the fact whether a unit is
loaded or not is invisible to clients. Use systemctl list-units --all
to comprehensively list all units currently loaded. Any unit for which none
of the conditions above applies is promptly unloaded. Note that when a unit
is unloaded from memory its accounting data is flushed out too. However,
this data is generally not lost, as a journal log record is generated
declaring the consumed resources whenever a unit shuts down.

Processes systemd spawns are placed in individual Linux control
groups named after the unit which they belong to in the private systemd
hierarchy. (see cgroups.txt[1] for more information about control
groups, or short "cgroups"). systemd uses this to effectively keep
track of processes. Control group information is maintained in the kernel,
and is accessible via the file system hierarchy (beneath
/sys/fs/cgroup/systemd/), or in tools such as systemd-cgls(1) or
ps(1) (ps xawf -eo pid,user,cgroup,args is particularly useful
to list all processes and the systemd units they belong to.).

systemd is compatible with the SysV init system to a large degree:
SysV init scripts are supported and simply read as an alternative (though
limited) configuration file format. The SysV /dev/initctl interface is
provided, and compatibility implementations of the various SysV client tools
are available. In addition to that, various established Unix functionality
such as /etc/fstab or the utmp database are supported.

systemd has a minimal transaction system: if a unit is requested
to start up or shut down it will add it and all its dependencies to a
temporary transaction. Then, it will verify if the transaction is consistent
(i.e. whether the ordering of all units is cycle-free). If it is not,
systemd will try to fix it up, and removes non-essential jobs from the
transaction that might remove the loop. Also, systemd tries to suppress
non-essential jobs in the transaction that would stop a running service.
Finally it is checked whether the jobs of the transaction contradict jobs
that have already been queued, and optionally the transaction is aborted
then. If all worked out and the transaction is consistent and minimized in
its impact it is merged with all already outstanding jobs and added to the
run queue. Effectively this means that before executing a requested
operation, systemd will verify that it makes sense, fixing it if possible,
and only failing if it really cannot work.

Note that transactions are generated independently of a unit's
state at runtime, hence, for example, if a start job is requested on an
already started unit, it will still generate a transaction and wake up any
inactive dependencies (and cause propagation of other jobs as per the
defined relationships). This is because the enqueued job is at the time of
execution compared to the target unit's state and is marked successful and
complete when both satisfy. However, this job also pulls in other
dependencies due to the defined relationships and thus leads to, in our our
example, start jobs for any of those inactive units getting queued as
well.

systemd contains native implementations of various tasks that need
to be executed as part of the boot process. For example, it sets the
hostname or configures the loopback network device. It also sets up and
mounts various API file systems, such as /sys or /proc.

For more information about the concepts and ideas behind systemd,
please refer to the Original Design Document[2].

Note that some but not all interfaces provided by systemd are
covered by the Interface Stability Promise[3].

Units may be generated dynamically at boot and system manager
reload time, for example based on other configuration files or parameters
passed on the kernel command line. For details, see
systemd.generator(7).

Systems which invoke systemd in a container or initrd environment
should implement the Container Interface[4] or initrd
Interface[5] specifications, respectively.

The systemd system manager reads unit configuration from
various directories. Packages that want to install unit files shall place them
in the directory returned by pkg-config systemd
--variable=systemdsystemunitdir. Other directories checked are
/usr/local/lib/systemd/system and /usr/lib/systemd/system. User configuration
always takes precedence. pkg-config systemd
--variable=systemdsystemconfdir returns the path of the system
configuration directory. Packages should alter the content of these
directories only with the enable and disable commands of the
systemctl(1) tool. Full list of directories is provided in
systemd.unit(5).

User unit directories

Similar rules apply for the user unit directories.
However, here the XDG Base Directory specification[6] is followed to
find units. Applications should place their unit files in the directory
returned by pkg-config systemd --variable=systemduserunitdir. Global
configuration is done in the directory reported by pkg-config systemd
--variable=systemduserconfdir. The enable and disable
commands of the systemctl(1) tool can handle both global (i.e. for all
users) and private (for one user) enabling/disabling of units. Full list of
directories is provided in systemd.unit(5).

SysV init scripts directory

The location of the SysV init script directory varies
between distributions. If systemd cannot find a native unit file for a
requested service, it will look for a SysV init script of the same name (with
the .service suffix removed).

SysV runlevel link farm directory

The location of the SysV runlevel link farm directory
varies between distributions. systemd will take the link farm into account
when figuring out whether a service shall be enabled. Note that a service unit
with a native unit configuration file cannot be started by activating it in
the SysV runlevel link farm.

Upon receiving this signal the systemd system manager
serializes its state, reexecutes itself and deserializes the saved state
again. This is mostly equivalent to systemctl daemon-reexec.

systemd user managers will start the exit.target unit when this
signal is received. This is mostly equivalent to systemctl --user start
exit.target --job-mode=replace-irreversible.

SIGINT

Upon receiving this signal the systemd system manager
will start the ctrl-alt-del.target unit. This is mostly equivalent to
systemctl start ctrl-alt-del.target --job-mode=replace-irreversible. If
this signal is received more than 7 times per 2s, an immediate reboot is
triggered. Note that pressing Ctrl+Alt+Del on the console will trigger this
signal. Hence, if a reboot is hanging, pressing Ctrl+Alt+Del more than 7 times
in 2 seconds is a relatively safe way to trigger an immediate reboot.

systemd user managers treat this signal the same way as
SIGTERM.

SIGWINCH

When this signal is received the systemd system manager
will start the kbrequest.target unit. This is mostly equivalent to
systemctl start kbrequest.target.

This signal is ignored by systemd user managers.

SIGPWR

When this signal is received the systemd manager will
start the sigpwr.target unit. This is mostly equivalent to systemctl start
sigpwr.target.

SIGUSR1

When this signal is received the systemd manager will try
to reconnect to the D-Bus bus.

SIGUSR2

When this signal is received the systemd manager will log
its complete state in human-readable form. The data logged is the same as
printed by systemd-analyze dump.

SIGHUP

Reloads the complete daemon configuration. This is mostly
equivalent to systemctl daemon-reload.

Enables display of status messages on the console, as
controlled via systemd.show_status=1 on the kernel command line.

SIGRTMIN+21

Disables display of status messages on the console, as
controlled via systemd.show_status=0 on the kernel command line.

SIGRTMIN+22

Sets the service manager's log level to
"debug", in a fashion equivalent to systemd.log_level=debug
on the kernel command line.

SIGRTMIN+23

Restores the log level to its configured value. The
configured value is derived from – in order of priority – the
value specified with systemd.log-level= on the kernel command line, or
the value specified with LogLevel= in the configuration file, or the
built-in default of "info".

SIGRTMIN+24

Immediately exits the manager (only available for --user
instances).

SIGRTMIN+26

Restores the log target to its configured value. The
configured value is derived from – in order of priority – the
value specified with systemd.log-target= on the kernel command line, or
the value specified with LogTarget= in the configuration file, or the
built-in default.

SIGRTMIN+27, SIGRTMIN+28

Sets the log target to "console" on
SIGRTMIN+27 (or "kmsg" on SIGRTMIN+28), in a fashion
equivalent to systemd.log_target=console (or
systemd.log_target=kmsg on SIGRTMIN+28) on the kernel command
line.

The value must be a boolean. Controls whether colorized
output should be generated. This can be specified to override the decision
that systemd makes based on $TERM and what the console is
connected to.

$SYSTEMD_URLIFY

The value must be a boolean. Controls whether clickable
links should be generated in the output for terminal emulators supporting
this. This can be specified to override the decision that systemd makes
based on $TERM and other conditions.

$LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES

Set by systemd for supervised processes during
socket-based activation. See sd_listen_fds(3) for more
information.

$NOTIFY_SOCKET

Set by systemd for supervised processes for status and
start-up completion notification. See sd_notify(3) for more
information.

For further environment variables understood by systemd and its
various components, see Known Environment Variables[7].

When run as system instance systemd parses a number of kernel command line
arguments[8]:

systemd.unit=, rd.systemd.unit=

Overrides the unit to activate on boot. Defaults to
default.target. This may be used to temporarily boot into a different boot
unit, for example rescue.target or emergency.service. See
systemd.special(7) for details about these units. The option prefixed
with "rd." is honored only in the initial RAM disk (initrd), while
the one that is not prefixed only in the main system.

systemd.dump_core

Takes a boolean argument or enables the option if
specified without an argument. If enabled, the systemd manager (PID 1) dumps
core when it crashes. Otherwise, no core dump is created. Defaults to
enabled.

systemd.crash_chvt

Takes a positive integer, or a boolean argument. Can be
also specified without an argument, with the same effect as a positive
boolean. If a positive integer (in the range 1–63) is specified, the
system manager (PID 1) will activate the specified virtual terminal (VT) when
it crashes. Defaults to disabled, meaning that no such switch is attempted. If
set to enabled, the VT the kernel messages are written to is selected.

systemd.crash_shell

Takes a boolean argument or enables the option if
specified without an argument. If enabled, the system manager (PID 1) spawns a
shell when it crashes, after a 10s delay. Otherwise, no shell is spawned.
Defaults to disabled, for security reasons, as the shell is not protected by
password authentication.

systemd.crash_reboot

Takes a boolean argument or enables the option if
specified without an argument. If enabled, the system manager (PID 1) will
reboot the machine automatically when it crashes, after a 10s delay.
Otherwise, the system will hang indefinitely. Defaults to disabled, in order
to avoid a reboot loop. If combined with systemd.crash_shell, the
system is rebooted after the shell exits.

systemd.confirm_spawn

Takes a boolean argument or a path to the virtual console
where the confirmation messages should be emitted. Can be also specified
without an argument, with the same effect as a positive boolean. If enabled,
the system manager (PID 1) asks for confirmation when spawning processes using
/dev/console. If a path or a console name (such as "ttyS0")
is provided, the virtual console pointed to by this path or described by the
give name will be used instead. Defaults to disabled.

systemd.service_watchdogs=

Takes a boolean argument. If disabled, all service
runtime watchdogs (WatchdogSec=) and emergency actions (e.g.
OnFailure= or StartLimitAction=) are ignored by the system
manager (PID 1); see systemd.service(5). Defaults to enabled, i.e.
watchdogs and failure actions are processed normally. The hardware watchdog is
not affected by this option.

systemd.show_status

Takes a boolean argument or the constant auto. Can
be also specified without an argument, with the same effect as a positive
boolean. If enabled, the systemd manager (PID 1) shows terse service status
updates on the console during bootup. auto behaves like false
until a unit fails or there is a significant delay in boot. Defaults to
enabled, unless quiet is passed as kernel command line option, in which
case it defaults to auto. If specified overrides the system manager
configuration file option ShowStatus=, see
systemd-system.conf(5). However, the process command line option
--show-status= takes precedence over both this kernel command line
option and the configuration file option.

Controls log output, with the same effect as the
$SYSTEMD_LOG_TARGET, $SYSTEMD_LOG_LEVEL,
$SYSTEMD_LOG_LOCATION, $SYSTEMD_LOG_COLOR environment variables
described above. systemd.log_color can be specified without an
argument, with the same effect as a positive boolean.

systemd.default_standard_output=,
systemd.default_standard_error=

Controls default standard output and error output for
services, with the same effect as the --default-standard-output= and
--default-standard-error= command line arguments described above,
respectively.

systemd.setenv=

Takes a string argument in the form VARIABLE=VALUE. May
be used to set default environment variables to add to forked child processes.
May be used more than once to set multiple variables.

systemd.machine_id=

Takes a 32 character hex value to be used for setting the
machine-id. Intended mostly for network booting where the same machine-id is
desired for every boot.

systemd.unified_cgroup_hierarchy

When specified without an argument or with a true
argument, enables the usage of unified cgroup hierarchy[9]
(a.k.a. cgroups-v2). When specified with a false argument, fall back to
hybrid or full legacy cgroup hierarchy.

If this option is not specified, the default behaviour is
determined during compilation (the -Ddefault-hierarchy= meson
option). If the kernel does not support unified cgroup hierarchy, the legacy
hierarchy will be used even if this option is specified.

systemd.legacy_systemd_cgroup_controller

Takes effect if the full unified cgroup hierarchy is not
used (see previous option). When specified without an argument or with a true
argument, disables the use of "hybrid" cgroup hierarchy (i.e. a
cgroups-v2 tree used for systemd, and legacy cgroup hierarchy[10],
a.k.a. cgroups-v1, for other controllers), and forces a full
"legacy" mode. When specified with a false argument, enables the use
of "hybrid" hierarchy.

If this option is not specified, the default behaviour is
determined during compilation (the -Ddefault-hierarchy= meson
option). If the kernel does not support unified cgroup hierarchy, the legacy
hierarchy will be used even if this option is specified.

quiet

Turn off status output at boot, much like
systemd.show_status=no would. Note that this option is also read by the
kernel itself and disables kernel log output. Passing this option hence turns
off the usual output from both the system manager and the kernel.

debug

Turn on debugging output. This is equivalent to
systemd.log_level=debug. Note that this option is also read by the
kernel itself and enables kernel debug output. Passing this option hence turns
on the debug output from both the system manager and the kernel.

emergency, rd.emergency, -b

Boot into emergency mode. This is equivalent to
systemd.unit=emergency.target or
rd.systemd.unit=emergency.target, respectively, and provided for
compatibility reasons and to be easier to type.

rescue, rd.rescue, single, s,
S, 1

Boot into rescue mode. This is equivalent to
systemd.unit=rescue.target or rd.systemd.unit=rescue.target,
respectively, and provided for compatibility reasons and to be easier to
type.

2, 3, 4, 5

Boot into the specified legacy SysV runlevel. These are
equivalent to systemd.unit=runlevel2.target,
systemd.unit=runlevel3.target, systemd.unit=runlevel4.target,
and systemd.unit=runlevel5.target, respectively, and provided for
compatibility reasons and to be easier to type.

Daemon status notification socket. This is an
AF_UNIX datagram socket and is used to implement the daemon
notification logic as implemented by sd_notify(3).

/run/systemd/private

Used internally as communication channel between
systemctl(1) and the systemd process. This is an AF_UNIX stream
socket. This interface is private to systemd and should not be used in
external projects.

/dev/initctl

Limited compatibility support for the SysV client
interface, as implemented by the systemd-initctl.service unit. This is a named
pipe in the file system. This interface is obsolete and should not be used in
new applications.

If run inside a Linux container these arguments may be passed as command
line arguments to systemd itself, next to any of the command line options
listed in the Options section above. If run outside of Linux containers,
these arguments are parsed from /proc/cmdline instead.